In oil and natural gas wells, casing is set into the well borehole. In the borehole, the casing is subjected to corrosive elements which act on the casing by reducing the casing wall thickness. Cement, which has been forced into the annulus between the casing and the surrounding strata, protects the outer surface of the casing from corrosion. But occasionally the bond between the cement and the casing is unsatisfactory, wherein the outer surface of the casing is exposed to corrosive elements. Furthermore, casing is typically cemented only for portions of its length, leaving exposed portions.
It is desirable to periodically survey the casing to determine the presence of corrosion and inside surface wear on the casing and the extent of damage. Corrosion (and wear) reduces the wall thickness of the casing, so by measuring the wall thickness, the presence or absence of corrosion can be inferred. Once corrosion has been detected on the casing, the situation can be evaluated and a decision can be made on whether or not to take corrective action, which corrective action is time consuming and expensive. In order to properly evaluate whether to take corrective action, the wall thickness of the corroded area, the extent of corrosion, and the location of the corrosion are factors which are considered.
In the prior art, there are apparatuses and methods for ultrasonically scanning casing to determine the presence of corrosion. However, the prior art fails to provide enough information on corrosion to make a proper evaluation. The prior art is exemplified by Zemanek, which U.S. Pat. No. 3,369,626 discloses an ultrasonic apparatus for use in scanning the inner surface of casing on the inner surface of an open well borehole. The ultrasonic apparatus, which is commercially known as the borehole televiewer, uses a rotating ultrasonic transducer housed inside of an acoustical window to provide full coverage of the casing, thus providing high areal resolution. The borehole televiewer is limited, however, to scanning he inner surface of casing, and thus cannot be used to determine corrosion occurring on the outer surface of the casing. Furthermore, the borehole televiewer uses analog signal processing, thereby limiting the capabilities of the apparatus.
Havira, U.S. Pat. No. 4,255,798 discloses an ultrasonic logging apparatus for evaluating the bond between the cement and the casing. The apparatus also determines the wall thickness of the casing for corrosion detection purposes. One version of the logging apparatus utilizes a single fixed transducer and a rotating reflector. The transducer has a bandwidth of 300 KHz-600 KHz, in order to excite the fundamental resonance of the casing wall and determine wall thickness. The relatively low bandwidth limits the resolution of the logging apparatus in determining casing wall thickness. Walls thinner than about 5mm are unresolvable. It is frequently desired to resolve thinner walls. Furthermore, the presence of the acoustical window in the path of the acoustic return causes distortion of the acoustic return, further limiting the resolution. The acoustical window could be impedance matched to reduce distortion, however varying conditions downhole, including mud weights and temperatures, make impedance matching of the acoustic return impractical.
Another version of the logging apparatus of Havira, U.S. Pat. No. 4,255,798 uses transducers which are directly exposed to the borehole fluids. Plural transducers are provided in a circumferential arrangement. Because transducers are fixed full coverage of the casing wall is not provided for, thus limiting the areal resolution of the logging apparatus.
What is needed is a method that measures wall thickness, which method has high casing wall thickness resolution and that provides full areal resolution of the casing wall, wherein corrosion on the casing wall can be detected.